1. Technical Field
The invention relates to polyimide resins, particularly to polyimide resins containing polymaleic anhydride.
2. Related Art
Thermal curing resin compositions have been developing by material industries for their excellent strength, temperature resistance and low water absorption. As thermal curing resins, polyimides are noted for their excellent temperature resistance. Thin films that manufactured by thermal curing polyimide resin compositions may be extensively in the fields of printed circuit boards, surface heating element, electromagnetic shield material and flat cable, etc.
Polyimides have excellent heat resistance, mechanical properties and chemical resistance. Therefore, the polyimides are widely used in the aerospace industry, electronic materials, etc. Now the polyimides that are generally in use are mostly aromatic polyimides. However, most of the aromatic polyimide cannot be dissolved in the solvent and is non-thermoplastic, and thus difficult to process. Polyamic acid that is precursor of polyimide can be dissolved in the solvent. Therefore, polyimide may be formed by forming a desired shape by the polyamic acid solution, and then imidization is carried out.
However, imidization is accompanied by water evaporation because the reaction temperature of thermal imidization has reached more than 300° C. that exceeds the boiling point of water. Accordingly, the disadvantage of wrinkled surface of the thick film formed of the polyimide resin by the thermal ring closure step will occur. The temperature for film forming is hard to select properly. On the other hand, the film formed of the polyamic acid fails to keep a property of excellent temperature resistance of the polyimide as the imidization is omitted. Also, polyamic acid solution is hard to preserve, because hydrolysis of the polyamic acid solution is easy to occur in presence of water.
Polyimides are used extensively in the electronic fields as insulation film or protective coating on semiconductor devices. Especially, aromatic polyimides play an important role for high density and multi-function of flexible printed circuit substrates and integrated circuits due to the excellent temperature resistance, mechanic strength and insulation property.
Accordingly, precursor solution of polyimides is typically used for the formation of interlayer insulation film or protective coating of micro-circuit. The precursor solution of polyimides such as polyamic acid (PAA) solution, polyamic acid acetate solution, polyamic acid trimethylsilyl acetate solution and polyamic acid bis(diethyl amide) solution may be formed by reacting diamine compounds with tetracarboxylic dianhydride. The precursor solutions of polyimides are all polymer solution with high degree of polymerization. Typically, the film of polyimides is formed by coating the polymer solution on a substrate such as copper or glass, and then heated to carry out imidization and remove the solvent.
However, it is required to reduce the concentration of solute for obtaining a proper viscosity of the polymer solution when coating the polymer solution with high degree of polymerization. On the other hand, in order to increase the production, it is required to increase the concentration of solute, and thus the polymer solution has an increased viscosity and is difficult for coating. Further, if polymers with low molecular weight are manufactured to obtain a proper viscosity of the polymer solution for coating, it is not able to form a film with excellent temperature resistance and mechanic strength. Moreover, the polymer solution is hard to preserve in a condition of maintaining the original degree of polymerization for a long time.